Hearing restoration or compensation devices, commonly known as hearing aids, provide a tremendous benefit to a patient with congenital hearing loss or whose hearing has deteriorated due to age, genetics, illness, or injury. There is a wide variety of commercially available devices that can be worn externally or can be implanted within the body of the patient.
The service life of an implantable medical device is often limited by the battery capacity of its internal battery. In order to increase the service of the device, the electronics in the device may be designed to reduce or minimize power consumption. For hearing aids, the output amplifier or amplifiers may consume a significant portion of the overall power dissipated in the device. There is ongoing effort to develop techniques that reduce power consumption by the output amplifier.
It is desirable for a hearing aid output amplifier to retain its linearity over the full range of normal human hearing, which is typically 20 Hz to 20 kHz, and over the full dynamic range, which can typically span 60 to 80 dB. Retaining linearity is most challenging for loud signals at high frequencies. In particular, it is desirable to avoid a condition referred to as “slew rate limiting”.
The slew rate at a particular point in a circuit describes how quickly the voltage must change with respect to time. One may think of a simple sine wave as an example, where the zero-crossing (being the point at which the voltage changes most quickly) places a particular requirement on the voltage change per time. As the sine wave amplitude increases, so does the slope at the zero crossing. Likewise, as the frequency of the sine wave increases, so does the slope at the zero crossing. The voltage-per-time requirements are most demanding when the sine wave simultaneously has a large amplitude and a high frequency.
Mathematically, the slew rate, or voltage change per time, requirements are linearly proportional to a current specification on the output amplifier. In other words, if the output amplifier can deliver a particular value of current, then the amplifier can provide the required voltage change per time at the most demanding conditions, which are loud volumes at high frequencies. This particular value of current is known as a “bias current”.
When power dissipation is not an issue, such as for guitar amplifiers or other devices that may be plugged into a wall, the bias current may be run at a constant value. However, running the bias current at a constant value is generally unacceptable in a hearing aid or other low-power electronic devices. Such a constant-valued current would consume electrical power even when it is not required, since there are many occasions when there are not loud volumes at high frequencies.
In addition, there is a condition known as “crossover distortion”. Basically, at voltages near a point where current is switched between a set of matched transistors, a kink may appear in the output voltage, which can lead to a “flattening out” of voltage at a zero crossing or at some other voltage level. Often, one can reduce or eliminate crossover distortion by increasing a bias current. Along with slew rate limiting, it is desirable to avoid crossover distortion.
Accordingly, there exists a need for a technique of delivering a required bias current to the output amplifier in a hearing aid, while consuming less hearing aid battery energy than a constant-level bias current would consume. Such a technique may also be used in other applications that use broadband amplifiers.